Only one in three people with dementia ever gets a formal diagnosis from a doctor, but the gradual, inexorable decline of memory and acuity will be all too visible to relatives and carers who see their loved ones slip away slowly in later life.

Evidence suggests that the main way to prevent dementia is to live an active life, both mentally and physically. But, as the cases of Iris Murdoch, Terry Pratchett and now Margaret Thatcher testify, degenerative disease can strike even the most intellectually formidable characters. Indeed, dementia used to be considered not so much a disease as a life stage.

Alzheimer’s is the most common cause of dementia, afflicting more than 700,000 Britons, but about one in 20 adults suffer problems with memory and concentration after the age of 65. After 80, the figure is one in five.

Stress and a lack of sleep, for which the former Prime Minister was famous, can affect memory in the short-term but there is nothing to suggest that they can cause the noticeable decline over years or decades, which starts with forgetting your keys and ends with forgetting what keys are for.

Dementia is characterised by changes in brain structure, tangles of protein and plaques, large and obvious physical changes that can disrupt some- one’s thoughts and personality.

In recent decades scientists have learnt a lot more about how the brain ages, and there are reasons for optimism. It is clear, for example, that more educated people suffer from Alzheimer’s less severely, or develop it at a greater age.

There is evidence that simple anti-inflammatory drugs may modify progress of the disease. Several emerging drugs target amyloid-beta, a protein in the brain, but some scientists doubt that this is the only factor at play. Research suggests that a direct attack on the tangles of “tau” protein, first identifed by Alois Alzheimer in 1906, could be more effective, slowing progress of the disease by as much as 80 per cent.

In the absence of an effective treatment, early diagnosis of dementia is vital. It enables people to plan for the future, manage their symptoms and make the most of now.

It may be some time before we can say that dementia is controllable, but within ten years we may have medicines capable of holding it at bay.

Explore Health

• Expert Advice

• Health Features

• Mental Health

• Alternative Medicine

• Child Health

• Health Club